The present invention relates to an organic electroluminescent device (hereinafter this will be referred to as an organic EL device) . More precisely, the invention relates to an organic EL device having the advantages of applicability to lightweight, thin and low-voltage driving displays, good luminescent capacity attenuating little even in long-term driving operation, and good durability.
As being self-luminescent, organic electroluminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, they are much used in various fields of thin-film display devices, back lights for liquid-crystal displays, flat light sources, etc.
Distributed electroluminescent devices are now in practical use. As they require alternating voltage of at least tens volts and 10 kHz or more, their driving circuits are complicated.
In the circumstances, organic EL devices capable of being driven at lowered voltage of 10 volts or so and capable of emitting high-luminance light are much studied these days. For example, thin-film organic EL devices having a multi-layered structure of transparent electrode/hole injection layer/emitting layer/back electrode are proposed in Appl. Phys. Lett., Vol. 51, pp. 913-915 (1987) by C. W. Tang and S.A. Van Slyke, and in Japanese Patent Laid-Open No. 264629/1988. These are so designed that the hole injection layer therein can efficiently inject holes into the emitting layer therein. The emitting layer in such organic EL devices may have a single-layered structure, which, however, could not enjoy well-balanced electron transportation and hole transportation. To solve the problem, the emitting layer is modified to have a multi-layered structure of improved performance.
However, the process of forming the multi-layered emitting layer is complicated and takes a lot of time. Another problem with it is that the multi-layered structure is against the recent tendency in the art which is toward reducing the thickness of layers constituting organic EL devices. On the other hand, down-sized, compact and portable information appliances are much desired these days, and they are required to be driven at low voltage. In the circumstances, various types of light-emitting materials and hole-transporting materials are tried for such lightweight, low-voltage driving appliances.
Further, the most important theme in practical studies of organic EL devices is to establish the technique of preventing the attenuation of the luminance of the devices in long-term driving and to provide practicable organic EL devices. In this respect, it is said that the purity of organic compounds to be used for producing constituent materials for organic EL devices has a great influence on the attenuation of the luminous efficiency and the luminance of the devices produced, for example, as in xe2x80x9cMonthly Display, Sept. 15, 1995xe2x80x9d, and xe2x80x9cApplied Physics, Vol. 66, No. 2, pp. 114-115, 1997xe2x80x9d. However, the influences of the structures and the properties of organic compounds to be used for producing organic EL devices on the properties of the devices produced are not as yet clarified, and no method has heretofore been established capable of quantitatively determining the influences in question.
In that situation, the object of the present invention is to provide an organic EL device having the advantages of applicability to lightweight, thin and low-voltage driving displays, good luminescent capacity attenuating little even in long-term driving operation, and good durability.
We, the present inventors have assiduously studied in order to attain the object as above, and, as a result, have found that the object can be attained by an organic EL device in which at least one organic compound layer comprises an organic compound material having an impurity concentration of smaller than 1000 ppm including 0 ppm. On the basis of this finding, we have completed the present invention.
Specifically, the invention is summarized as follows:
(1) An organic electroluminescent device that comprises organic compound layer(s) including at least one organic emitting layer sandwiched between a pair of electrodes, wherein at least one organic compound layer is formed from an organic compound material having an impurity concentration of lower than 1000 ppm.
(2) An organic electroluminescent device that comprises organic compound layer(s) including at least one organic emitting layer sandwiched between a pair of electrodes, wherein at least one organic compound layer is formed from an organic compound material having an impurity concentration of lower than 500 ppm and the impurity therein is a halogen-containing compound.
(3) The organic electroluminescent device of above (2), wherein the halogen-containing compound is a halogen compound.
(4) The organic electroluminescent device of any of above (1) to (3), wherein the organic compound layers are a hole injection layer, an organic emitting layer and an electron injection layer.
(5) The organic electroluminescent device of any of above (1) to (4), wherein at least one organic compound material to form the organic compound layer(s) is purified through sublimation.
(6) The organic electroluminescent device of any of above (1) to (4), wherein at least one organic compound material to form the organic compound layer(s) is purified through recrystallization or reprecipitation, or through recrystallization combined with reprecipitation.
(7) A method for selecting organic compound materials for organic electroluminescent devices, comprising determining, through high-performance liquid chromatography, the impurity content of each organic compound material to form organic compound layers for the devices, selecting those having an impurity content of smaller than 1000 ppm out of the materials analyzed, and using the thus-selected materials for forming the organic compound layers.
(8) A method for selecting organic compound materials for organic electroluminescent devices, comprising determining the impurity content of at least one organic compound material to form organic compound layers for the devices, selecting those having an impurity content of smaller than 1000 ppm out of the materials analyzed, and using the thus-selected materials for forming the organic compound layers.
(9) The method of above (7) or (8) for selecting organic compound materials for organic electroluminescent devices, wherein the impurity in the organic compound materials is a halogen-containing compound.
Modes of carrying out the invention are described hereinunder.
The invention is an organic EL device that comprises organic compound layer (s) including at least one organic emitting layer sandwiched between a pair of electrodes, wherein at least one organic compound layer is formed from an organic compound material having an impurity concentration of lower than 1000 ppm (by weight, but, as the case may be, by volume).
The organic EL device of the invention comprises organic compound layer(s) including at least one organic emitting layer sandwiched between a pair of electrodes. Its typical structures are mentioned below, but are not limitative.
 less than 1 greater than  Anode/emitting layer/cathode
 less than 2 greater than  Anode/hole injection layer/emitting layer/cathode
 less than 3 greater than  Anode/emitting layer/electron injection layer/cathode
 less than 4 greater than  Anode/hole injection layer/emitting layer/electron injection layer/cathode
 less than 5 greater than  Anode/organic semiconductor layer/emitting layer/cathode
 less than 6 greater than  Anode/organic semiconductor layer/electron barrier layer/emitting layer/cathode
 less than 7 greater than  Anode/organic semiconductor layer/emitting layer/adhesion improving layer/cathode
 less than 8 greater than  Anode/hole injection layer/hole transporting layer/emitting layer/electron injection layer/cathode
The organic EL device of the invention may have any of these layer structures, but preferably has the layer structure  less than 8 greater than . The organic compound layers referred to herein include the emitting layers and others sandwiched between the anode and the cathode in the layer structures mentioned above. In the organic EL device of the invention, at least any one of these organic compound layers is formed from an organic compound material having an impurity concentration of lower than 1000 ppm.
The organic EL device is formed on a transparent substrate. The transparent substrate supports the organic EL device, and its transparency in a visible ray range of from 400 to 700 nm is preferably at least 50%. Also preferably, the substrate is flat and has a smooth surface.
Preferred examples of the transparent substrate of that type are glass plates, synthetic resin plates, etc. The glass plates may be made of, for example, soda-lime glass, barium-strontium glass, lead glass, aluminosilicate glass, borosilicate glass, barium borosilicate glass, quartz, etc. The synthetic resin plates may be made of, for example, polycarbonate resins, acrylic resins, polyethylene terephthalate resins, polyether sulfide resins, polysulfone resins, etc.
For the electrode material for the anode, preferred are metals, alloys, electroconductive materials and their mixtures having a large work function (at least 4 eV). Specific examples of the electrode material are metals such as Au, etc.; and electroconductive materials such as CuI, ITO, SnO2, ZnO, etc. To form the anode, the electrode material is formed into a thin film through vapor deposition, sputtering or the like. For its properties, it is desirable that the anode through which the light from the emitting layer is taken out has a light transmittance of larger than 10%. Also preferably, the sheet resistance of the anode is at most hundreds Q/square. Though depending on its material, the anode has a thickness generally falling between 10 nm and 1 xcexcm, but preferably between 10 nm and 200 nm.
It is desirable that the emitting layer in the organic EL device of the invention has all the following functions.
 less than 1 greater than  Hole and electron receiving function: This is to receive holes from anodes and hole injection layers and receive electrons from cathodes and electron injection layers while in an electric field.
 less than 2 greater than  Hole and electron transporting function: This is to move the injected charges (electrons and holes) by the force of the electric field.
 less than 3 greater than  Light emitting function: This is to provide a site for electron-hole recombination to emit light.
In the anode, the degree of hole injection may differ from that of electron injection, and the degree of hole transportation and that of electron transportation that are represented by hole and electron mobility may also differ from each other. Preferably, any one type of the charges is moved in the anode.
The light-emitting material in the organic EL device is principally an organic compound. Concretely, it includes the following compounds, any of which are used in the device depending on the desired color tone.
For example, for UV to violet emission, compounds of the following general formula [1] are preferred. 
wherein X represents a group of the following general formula [2]: 
and Y represents a group of the following general formula [3]: 
For the phenyl, phenylene and naphthyl groups in the compounds of formula [1], usable are compounds having one or more substituents such as an alkyl group having from 1 to 4 carbon atoms, an alkoxy group having from 1 to 4 carbon atoms, a hydroxyl group, a sulfonyl group, a carbonyl group, an amino group, a dimethylamino group, a diphenylamino group, etc. A plurality of these substituents, if any, may be bonded to each other to form a saturated 5-membered or 6-membered ring. Regarding their configuration, the phenyl, phenylene and naphthylene groups in the compounds are preferably para-positioned, since the bonding stability is good and since the compounds can be easily formed into flat and smooth films through vapor deposition. Specific examples of the compounds of formula [1] are mentioned below. 
Of these compounds, especially preferred are p-quaterphenyl derivatives and p-quinquephenyl derivatives.
For blue to green emission, for example, employable are benzothiazole-type, benzimidazole-type, benzoxazole-type and other fluorescent brightening compounds, metal-chelated oxinoide compounds and styrylbenzene compounds. For these, for example, referred to are the compounds described Japanese Patent Laid-Open No. 194393/1984. Still other compounds usable herein are listed in Chemistry of Synthetic Dyes, 1971, pp. 628-637, and 640.
For the chelated oxinoide compounds, for example, usable are the compounds described in Japanese Patent Laid-Open No. 295695/1988. Typically, preferred are 8-hydroxyquinoline metal complexes such as tris(8-quinolinol)aluminium, etc., and also dilithiumepinetridione, etc.
For the styrylbenzene compounds, for example, usable are the compounds described in European Patents 0319881 and 0373582. Also usable for the material for the emitting layer are distyrylpyrazine derivatives such as those described in Japanese Patent Laid-Open No. 252793/1990. Apart from these, still usable for the material for the emitting layer are polyphenyl compounds such as those described in European Patent 0387715.
In addition to the fluorescent brightening agents, metal-chelated oxinoide compounds, styrylbenzene compounds and others mentioned above, further usable for the material for the emitting layer are, for example, 12-phthaloperinone (in J. Appl. Phys., Vol. 27, L 713, 1988), 1,4-diphenyl-1,3-butadiene, 1,1,4,4-tetraphenyl-1,3-butadiene (both in Appl. Phys. Lett., Vol. 56, L 799, 1990), naphthalimide derivatives (in Japanese Patent Laid-Open No. 305886/1990), perylene derivatives (in Japanese Patent Laid-Open No. 189890/1990), oxadiazole derivatives (in Japanese Patent Laid-Open No. 216791/1990, or the oxadiazole derivatives disclosed by Hamada et al. in the 38th Applied Physics-related Joint Lecture Meeting), aldazine derivatives (in Japanese Patent Laid-Open No. 220393/1990) pyrazoline derivatives (in Japanese Patent Laid-Open No. 220394/1990), cyclopentadiene derivatives (in Japanese Patent Laid-Open No. 289675/1990), pyrrolopyrole derivatives (in Japanese Patent Laid-Open No. 296891/1990), styrylamine derivatives (Appl. Phys. Lett., Vol. 56, L 799, 1990), coumarin compounds (in Japanese Patent Laid-Open No. 191694/1990), polymer compounds such as those described in International Patent Publication WO90/13148 and Appl. Phys. Lett., Vol. 58, 18, P 1982, 1991, as well as 9,9xe2x80x2,10,10xe2x80x2-tetraphenyl-2,2xe2x80x2-bianthracenes, PPV (polyparaphenylenevinylene) derivatives, polyfluorene derivatives and their polymers and others, for example, those having the following structures: 
and also 9,10-bis(N-(4-(2-phenylvinyl-1-yl)phenyl-N-phenylamino) anthracene, etc. Moreover, phenylanthracene derivatives of the following formula, such as those described in Japanese Patent Laid-Open No. 12600/1996 are also usable for light-emitting materials.
A1xe2x80x94Lxe2x80x94A2
wherein A1 and A2 each represent a monophenylanthryl or diphenylanthryl group, and these may be the same or different; and L represents a single bond or a divalent linking group.
Especially preferred are phenylanthracene derivatives of the following general formulae: 
wherein R1 and R2 each represent an alkyl group, a cycloalkyl group, an aryl group, an alkenyl group, an alkoxy group, an aryloxy group, an amino group, or a heterocyclic group, and they may be the same or different; r1 and r2 each indicate 0 or an integer of from 1 to 5; when r1 and r2 each are an integer of 2 or more, the groups of R1""s and R2""s each may be the same or different, and R1""s and R2""s may be bonded to each other to form a ring; L1 represents a single bond or an arylene group, and the arylene group may be interrupted by an alkylene group, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NRxe2x80x94 (where R indicates an alkyl or aryl group) existing therein; R3 and R4 each represent an alkyl group, a cycloalkyl group, an aryl group, an alkenyl group, an alkoxy group, an aryloxy group, an amino group, or a heterocyclic group, and they may be the same or different; r3 and r4 each indicate 0 or an integer of from 1 to 5; when r3 and r4 each are an integer of 2 or more, the groups of R3""s and R4""s each may be the same or different, and R3""s and R4""s may be bonded to each other to form a ring; L2 represents a single bond or an arylene group, and the arylene group may be interrupted by an alkylene group, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NRxe2x80x94 (where R indicates an alkyl or aryl group) existing therein.
Specific examples of such anthracenes or phenylanthracenes are described below. 
In these formulae, R11 to R45 each represent an alkyl group, a cycloalkyl group, an aryl group, an alkenyl group, an alkoxy group, an aryloxy group, an amino group, or a heterocyclic group; and they may be the same or different.
Further mentioned are the following compounds: 
Naphthacene derivatives such as those mentioned below are also usable for light-emitting materials. 
wherein X represents a cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthio group, a substituted or unsubstituted arylthio group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted amino group, or a substituent for rubrene derivatives; i indicates an integer of from 1 to 28; and X""s may be the same or different.
Amine compounds such as those mentioned below are also usable for light-emitting materials. 
wherein A1 to A4 each independently represent an aryl group having from 6 to 16 carbon atoms; and R1 to R8 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, an aryl group or an amino group. 
wherein A and B each represent an optionally-substituted aromatic ring. 
wherein A, B, C and D each present a substituted or unsubstituted alkyl group, a unsubstituted or unsubstituted monocyclic group, or a substituted or unsubstituted, condensed polycyclic group; and A and B, or C and B may together form a heterocyclic group along with the nitrogen atom that bonds to the skeleton. 
Other amine compounds also usable for light-emitting material are mentioned below. 
wherein A1 to A4 each independently represent an aryl group having from 6 to 16 carbon atoms, and the aryl group may be substituted with a hydrogen atom, an alkyl group, an alkoxy group, an aryl group or an amino group; and A represents a single bond, or an optionally-substituted arylene or polyarylene group.
Aromatic dimethylidene compounds (such as those described in European Patent 0388768 and Japanese Patent Laid-Open No. 231970/1991) are also usable for materials for emitting layers. In general, they are represented by the following formula: 
wherein Ar represents an arylene or polyarylene group; R1 to R3 each represent a hydrogen atom, an alkyl group or an aryl group; and n indicates an integer of from 1 to 6.
The aryl group includes a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an anthryl group, a phenanthryl group, a pyrenyl group, a chrysenyl group, a fluorenyl group, etc. The arylene group includes a phenylene group, a biphenylene group, a terphenylene group, a naphthylene group, an anthrylene group, a phenanthrylene group, a pyrenylene group, a chrysenylene group, a f luorenylene group, etc. Preferably, the arylene group contains an anthracene skeleton, including, for example, an anthrylene group, a diphenylanthrylene group, a bianthrylene group, etc. R1 is preferably a hydrogen atom. Specific examples of the compounds are 4,4xe2x80x2-bis(2,2-di-t-butylphenylvinyl)biphenyl, 4,4xe2x80x2-bis(2,2-diphenylvinyl)biphenyl, 4,4xe2x80x3-bis(2,2-diphenylvinyl)-p-terphenyl, 9,10-bis(4xe2x80x2-(2,2-diphenylvinyl)biphenyl)anthracene, 9,10-(4-(2,2-diphenylvinyl)phenyl)anthracene, 9,9xe2x80x2-(4-(2,2-diphenylvinyl)phenyl)-10,10xe2x80x2-bianthracene, and their derivatives.
In addition to the above, also usable herein are styryl group-having light-emitting materials. For these, preferred are styrylamine compounds of the following formula: 
wherein Ar1 to Ar4 each represent an aryl group, and at least one of these is substituted with the following styryl group: 
where R1 to R3 each represent a hydrogen atom, an alkyl group, or an aryl group.
The aryl group for Ar1 to Ar4 and R1 to R3 includes a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an anthryl group, a phenanthryl group, a pyrenyl group, a chrysenyl group, a fluorenyl group, etc. R1 is preferably a hydrogen atom. A indicates a linking group, representing a single bond, an arylene group or a polyarylene group. It includes a phenylene group, a biphenylene group, an anthrylene group, a phenanthrylene group, a pyrenylene group, a chrysenylene group, a diphenylanthrylene group, and also groups having any of the following structural formulae: 
wherein Ar5 and Ar6 each represent an aryl group, like Ar1 to Ar1.
Also usable herein for light-emitting materials are compounds of a general formula, (Rsxe2x80x94Q)2xe2x80x94Alxe2x80x94Oxe2x80x94L wherein L represents a hydrocarbon group containing a phenyl skeleton and having from 6 to 24 carbon atoms; O-L indicates a phenolato ligand; Q represents a substituted 8-quinolinolato ligand; Rs represents a specifically-selected substituent to be on the 8-quinolinolato ring, and it stereospecifically governs the structure of the compounds so that more than two substituted 8-quinolinolato ligands do not bond to the aluminiumatom. The compounds are described in Japanese Patent Laid-Open No. 258862/1993. Concretely, they include bis(2-methyl-8-quinolinolato) (para-phenylphenolato) aluminium(III), bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminium(III), etc.
In addition to the above, also employable herein is a doping method for efficient mixed emission of blue and green, as in Japanese Patent Laid-Open No. 9953/1994, etc. In the method, the host is any of the above-mentioned light-emitting materials, and the dopant is a fluorescent dye of high blue to green emission. For example, the dopant is a coumarin dye or any other fluorescent dye that may be selected from fluorescent dyes serving as the host. Concretely, the host is a light-emitting material having a distyrylarylene skeleton, preferably 4,4xe2x80x2-bis(2,2-diphenylvinyl)biphenyl, and the dopant is a diphenylaminovinylarylene, preferably N,N-diphenylaminovinylbenzene.
The emitting layer for white emission is not specifically defined, and may be any of the following:
 less than 1 greater than  A layer that defines the energy level of the layers constituting a multi-layered organic EL structure, and emits light through tunnel injection thereinto (European Patent 0390551);
 less than 2 greater than  A layer that emits light through tunnel injection thereinto, like  less than 1 greater than  (Japanese Patent Laid-Open No. 230584/1991xe2x80x94its Examples demonstrate white-emitting devices);
 less than 3 greater than  A two-layered emitting layer (Japanese Patent Laid-Open Nos. 220390/1990 and 216790/1990);
 less than 4 greater than  A layer partitioned into a plurality of sections, in which the light-emitting materials for the respective sections differ in the wavelength range within which they emit light (Japanese Patent Laid-Open No. 51491/1992);
 less than 5 greater than  A stacked layer composed of a blue-emitting layer. (having a fluorescent peak that falls between 380 and 480 nm) and a green-emitting layer (having a fluorescent peak that falls between 480 and 580 nm), and further containing a red-emitting fluorescent material (Japanese Patent Laid-Open No. 207170/1994);
 less than 6 greater than  A combination of a blue-emitting layer that contains a blue-emitting fluorescent dye and a green-emitting layer that contains a red-emitting fluorescent dye, which further contains a green-emitting fluorescent material (Japanese Patent Laid-Open No. 142169/1995).
Of these, especially preferred is the structure  less than 5 greater than .
For the red-emitting fluorescent material, preferred are the following compounds: 
Next described is a method for forming an emitting layer from any of the materials mentioned above. Any known methods are applicable to emitting layer formation, including, for example, vapor deposition, spin coating, LB film formation, etc. For the emitting layer, especially preferred is a molecular deposition film. The molecular deposition film referred to herein is a thin film formed through vapor deposition of a material compound of being in a vapor phase condition, or a film formed through solidification of a material compound of being in a solution condition or in a liquid phase condition. In general, the molecular deposition film of that type could be differentiated from a thin film (molecular built-up film) formed in a method of LB film formation, because of the difference therebetween in the aggregation structure and the high-order structure and of the functional difference therebetween resulting from it.
Another method is employable for forming the emitting layer, which comprises dissolving a material compound in a solvent along with a binder such as resin or the like to prepare a solution, followed by filming the resulting solution into a thin film through spin coating or the like, as in Japanese Patent Laid-Open No. 51781/1982.
The thickness of the emitting layer thus formed in the manner mentioned above is not specifically defined. Depending on the condition, the thickness of the layer may be suitably varied, but preferably falls between 5 nm and 5 xcexcm. The emitting layer may have a single-layered structure comprising one or more of the materials mentioned above, or may have a multi-layered structure having additional emitting layer(s) of different compound(s).
Next described is the hole injection/transporting layer to be in the organic EL device of the invention. The layer is to assist hole injection into the emitting layer, transporting holes to the emitting region of the emitting layer. Its hole mobility is high, but its ionization energy is small, generally at most 5.5 eV. For the hole injection/transporting layer, preferred is a material capable of transporting holes to the emitting layer at a lower field strength. More preferably, the hole mobility in the layer is at least 10xe2x88x926 cm2/Vxc2x7sec, for example, in an electric field falling between 104 and 106 V/cm. The material mixed with an aromatic hydrocarbon compound for use herein to form the hole injection/transporting layer is not specifically defined, so far as the layer formed could have the preferred properties mentioned above. The material for the layer may be selected from any conventional photoconductive materials ordinarily used for hole-transporting materials, and also from any known materials ordinarily used for hole injection layers in organic EL devices.
Concretely, the material for forming the hole injection/transporting layer includes, for example, triazole derivatives (see U.S. Pat. No. 3,112,197, etc.), oxadiazole derivatives (see U.S. Pat. No. 3,189,447, etc.), imidazole derivatives (see Japanese Patent Publication No. 16096/1962, etc.), polyarylalkane derivatives (see U.S. Pat. Nos. 3,615,402, 3,820,989, 3,542,544, Japanese Patent Publication Nos. 555/1970, 10983/1976, Japanese Patent Laid-Open Nos. 93224/1976, 17105/1980, 4148/1981, 108667/1980, 156953/1980, 36656/1981, etc.), pyrazoline derivatives and pyrazolone derivatives (see U.S. Pat. Nos. 3,180,729, 4,278,746, Japanese Patent Laid-Open Nos. 88064/1980, 88065/1980, 105537/1974, 51086/1980, 80051/1981, 88141/1981, 45545/1982, 112637/1979, 74546/1980, etc.), phenylenediamine derivatives (see U.S. Pat. No. 3,615,404, Japanese Patent Publication Nos. 10105/1976, 3712/1971, 25336/1972, Japanese Patent Laid-Open Nos. 53435/1979, 110536/1979, 119925/1979, etc.), arylamine derivatives (see U.S. Pat. Nos. 3,567,450, 3,180,703, 3,240,597, 3,658,520, 4,232,103, 4,175,961, 4,012,376, Japanese Patent Publication Nos. 35702/1974, 27577/1964, Japanese Patent Laid-Open Nos. 144250/1980, 119132/1981, 22437/1981, German Patent 1,110,518, etc.), amino-substituted chalcone derivatives (see U.S. Pat. No. 3,526,501, etc.), oxazole derivatives (such as those described in U.S. Pat. No. 3,257,203, etc.), styrylanthracene derivatives (see Japanese Patent Laid-Open No. 46234/1981, etc.), fluorenone derivatives (see Japanese Patent Laid-Open No. 110837/1979, etc.), hydrazone derivatives (see U.S. Pat. No. 3,717,462, Japanese Patent Laid-Open Nos. 59143/1979, 52063/1980, 52064/1980, 46760/1980, 85495/1980, 11350/1982, 148749/1982, 311591/1990, etc.), stilbene derivatives (see Japanese Patent Laid-Open Nos. 210363/1986, 228451/1986, 14642/1986, 72255/1986, 47646/1987, 36674/1987, 10652/1987, 30255/1987, 93455/1985, 94462/1985, 174749/1985, 175052/1985, etc.), silazane derivatives (see U.S. Pat. No. 4,950,950), polysilane derivatives (see Japanese Patent Laid-Open No. 204996/1990), aniline-based copolymers (see Japanese Patent Laid-Open No. 282263/1990), electroconductive high-molecular weight oligomers (especially thiophene oligomers) described in Japanese Patent Laid-Open No. 211399/1989, etc.
For the materials for the hole injection/transporting layer, usable are those mentioned above. In addition, also usable are porphyrin compounds (such as those described in Japanese Patent Laid-Open No. 295695/1988, etc.), aromatic tertiary amine compounds and styrylamine compounds (see U.S. Pat. No. 4,127,412, Japanese Patent Laid-Open Nos. 27033/1978, 58445/1979, 149634/1979, 64299/1979, 79450/1980, 144250/1980, 119132/1981, 295558/1986, 98353/1986, 295695/1988, etc.), and other aromatic tertiary amine compounds.
Further usable are compounds having two condensed aromatic rings in the molecule such as those described in U.S. Pat. No. 5,061,569, e.g., 4,4xe2x80x2-bis(N-(1-naphthyl)-N-phenylamino)biphenyl; 4,4xe2x80x2,4xe2x80x3-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine with three triphenylamine units being starburst-wise bonded to the center nitrogen atom, such as that described in Japanese Patent Laid-Open No.308688/1992, etc. In addition, the aromatic dimethylidene compounds mentioned hereinabove for the materials for the emitting layer, and inorganic compounds such as p-type Si, p-type SiC and others are also usable for the materials for the hole injection/transporting layer.
For forming the hole injection/transporting layer, any of the compounds mentioned above is filmed into a thin film in any known method of, for example, vacuum evaporation, spin coating, casting, LB film formation, etc. The thickness of the hole injection/transporting layer is not specifically defined, generally falling between 5 nm and 5 xcexcm. If containing an aromatic hydrocarbon compound for use herein in its hole transporting zone, the hole injection/transporting layer may have a single-layered structure comprising one or more of the materials mentioned above, or may have a multi-layered structure having additional hole injection/transporting layer(s) of different compound(s).
Next described is the organic semiconductor layer. This is to assist hole injection or electron injection into the emitting layer, and preferably has en electroconductivity of at least 10xe2x88x9210 S/cm. Examples of the material for the organic semiconductor layer are electroconductive oligomers such as thiophene-containing oligomers, arylamine-containing oligomers described in Japanese Patent Laid-Open No. 193191/1996, etc.; electroconductive dendrimers such as arylamine-containing dendrimers, etc.
Next described is the electron injection layer. This is to assist electron injection into the emitting layer, and has high electron mobility. When formed from a material of good adhesion to cathodes, the electron injection layer serves also as an adhesion improving layer. For the materials for the electron injection layer, preferred are metal complexes of 8-hydroxyquinoline or its derivatives. Specific examples of metal complexes of 8-hydroxyquinoline or its derivatives that serve as electron-injecting materials for use herein are metal-chelated oxinoid compounds having an oxine (generally 8-quinolinol or 8-hydroxyquinoline) chelate, such as tris(8-quinolinol)aluminium.
Oxadiazole derivatives that serve as electron-transmitting compounds are also usable herein. Typically, they are represented by any of the following general formulae [4] to [6]: 
wherein Ar1, Ar2, Ar3, Ar5, Ar6 and Ar9 each independently represent a substituted or unsubstituted aryl group, and these may be the same or different; Ar4, Ar7 and Ar8 each independently represent a substituted or unsubstituted arylene group, and these may be the same or different.
The aryl group in these formulae [4] to [6] includes a phenyl group, a biphenyl group, an anthranyl group, a perylenyl group, a pyrenyl group. The arylene group therein includes a phenylene group, a naphthylene group, a biphenylene group, an anthranylene group, a perylenylene group, a pyrenylene group, etc. The substituents for these groups include an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a cyano group, etc. Of the electron-transmitting compounds, preferred are those capable of being readily filmed into thin films.
Specific examples of the electron-transmitting compounds are mentioned below. 
Next described is the cathode. For the electrode material for the cathode, usable are metals, alloys, electroconductive compounds and their mixtures having a small work function (at most 4 eV). Specific examples of the electrode material are sodium, sodium-potassium alloys, magnesium, lithium, magnesium-silver alloys, aluminium/aluminium oxide, aluminium-lithium alloys, indium, rare earth metals, etc.
To form the cathode, the electrode material is formed into a thin film through vapor deposition, sputtering or the like.
In case where the light from the emitting layer is taken out through the cathode, it is desirable that the cathode has a light transmittance of larger than 10%. Also preferably, the sheet resistance of the cathode is at most hundreds xcexa9/square. The thickness of the cathode generally falls between 10 nm and 1 xcexcm, but preferably between 50 nm and 200 nm.
Next described is a method for fabricating the organic EL device of the invention. Using the materials mentioned above and according to the processes also mentioned above, an anode, an emitting layer, and optionally a hole injection layer, and further optionally an electron injection layer are formed in that order, and a cathode is finally formed. Opposite to this order, a cathode is first formed and an anode is formed last to finish the organic EL device of the invention.
Described hereinunder is one embodiment of fabricating the organic EL device of the invention which has a structure of anode/hole injection layer/emitting layer/electron injection layer/cathode formed on a transparent substrate in that order.
A thin film of an anode material having a thickness of at most 1 xcexcm, preferably from 10 to 200 nm is first formed on a suitable transparent substrate through vapor deposition or sputtering. This serves as an anode. Next, a hole injection layer is formed on the anode. To form it, employable is any of vacuum evaporation, spin coating, casting, LB film formation or the like as so mentioned hereinabove, but preferred is vacuum evaporation as ensuring homogeneous films with few pin holes.
In case where the hole injection layer is formed through vacuum evaporation, the condition for vapor deposition varies, depending on the compound used (for the material for the hole injection layer), and on the crystal structure and the recombination structure of the hole injection layer to be formed. In general, it is desirable that the temperature of the vapor source falls between 50 and 450xc2x0 C., the vacuum degree falls between 10xe2x88x927 and 10xe2x88x923 Torr, the deposition rate falls between 0.01 and 50 nm/sec; the substrate temperature falls between xe2x88x9250 and 300xc2x0 C., the thickness of the film formed falls between 5 nm and 5 xcexcm.
Next, an emitting layer is formed on the hole injection layer. For forming it, a desired organic light-emitting material is filmed into a thin film through vacuum evaporation, sputtering, spin coating, casting or the like. Preferred is vacuum evaporation as ensuring homogeneous films with few pin holes. In case where the emitting layer is formed through vacuum evaporation, the condition for vapor deposition varies, depending on the compound used. In general, the condition for hole injection layer formation mentioned above could apply also to the emitting layer formation.
Next, an electron injection layer is formed on the emitting layer. Like the hole injection layer and the emitting layer, the electron injection layer must also be made of a homogeneous film, for which, therefore, preferred is vacuum evaporation. The condition for vapor deposition to form the electron injection layer may also be similar to that to form the hole injection layer and the emitting layer.
Finally, a cathode is formed on the electron injection layer to finish the intended organic EL device. The cathode is formed from a metal, for which is employable vapor deposition or sputtering. However, in order not to damage the underlying organic layers, preferred is vacuum evaporation.
The process of fabricating the organic EL device mentioned above is preferably achieved in one and the same vacuum chamber in which the degree of vacuum is not changed throughout the process of forming all the layers, from the anode to the cathode.
In case where a direct current voltage is applied to the organic EL device thus produced in the manner as above, a voltage of from 5 to 40 V may be applied thereto with its anode being charged to be plus (+) and its cathode to be minus (xe2x88x92), whereby the device emits light. Even if the same voltage is applied to the device in the reversed manner relative to the polarity of the electrodes, the device emits no light. On the other hand, in case where an alternating current is applied to the device, the device emits light only when its anode is charged to be plus (+) and its cathode to be minus (xe2x88x92). The wave mode of the alternating current to be applied to the device may be any desired one.
In the invention, at least one, but preferably at least two organic compounds for forming the organic compound layers of the organic EL device having the constitution as above have an impurity concentration of lower than 1000 ppm. In that condition, the organic compounds layers of the device are formed. More preferably, all the organic compound layers of the device have an impurity concentration of lower than 1000 ppm.
Purification to prepare such high-purity organic compounds is not specifically defined, and may be effected through sublimation, recrystallization, re-precipitation, zone melting, column purification, adsorption or the like. If desired, these purification methods may be combined in any desired manner. Of those, preferred is recrystallization for obtaining the intended high-purity organic compounds. For purifying sublimable compounds, preferred is sublimation. For purifying sublimable compounds through sublimation, it is desirable that the sublimation boat is first kept at a temperature lower than the sublimation point of the compound to be purified so as to previously remove the sublimable impurities from the compound before the compound is sublimed. Also preferably, the zone in which the sublimed compound is collected is designed to have a temperature gradient therethrough so that the sublimed mixture could be fractionated into the intended product and the impurities. The sublimation purification mode mentioned above is for purifying a compound by removing impurities from it, and is applicable to the invention.
Also preferably, organic compounds in which the oxygen and nitrogen atoms are directly or indirectly bonded to the xcfx84-conjugated carbon-carbon main chain (these are referred to as chelate complex compounds) are purified through sublimation. For sublimation purification, known are a stirring method and a shaking method. The stirring method includes (A) a mechanical stirring method in which lumps of an aggregated organic compound are directly crushed and milled with a man-powered or machine-powered stirring blade fitted to the tip of a stirrer sealed in vacuum, and stirring the system is continued so that the milled power is prevented from again aggregating to form lumps; (B) a magnetic stirring method in which is used a magnetic bar for milling an organic compound to be purified through sublimation in such a manner that the magnetic bar is rotated at a desired speed of rotation by the use of an external rotating machine to thereby directly crush and mill the lumps of the organic compound, and stirring the system is continued so that the milled power is prevented from again aggregating to form lumps; and (C) a method of dropping metal balls such as iron balls or the like onto the lumps of an organic compound to thereby directly crush and mill the lumps. Any of these methods is employable herein.
The shaking method includes (a) an ultrasonic shaking method in which an organic compound to be purified through sublimation is put into a container and exposed to ultrasonic waves from an external ultrasonic wave generator, thereby crushing and milling the lumps of the organic compound; and (b) a direct shaking method in which an organic compound to be purified through sublimation is put into a container, and the container is directly shaken by an external shaking machine fitted to the container or by human power applied thereto to thereby crush and mill the lumps of the organic compound. Any of these methods is employable herein.
The organic compound of which the impurity content has been reduced to smaller than 1000 ppm in the manner mentioned above is used for forming at least one organic compound layer of the organic EL device of the invention.
The impurities that may be in the organic compound materials to be formed into the organic compound layers of the device of the invention are generally derived from the starting substances used for preparing the organic compound materials, further including intermediates and precursors produced in the process of preparing the organic compounds (some intermediates and precursors produced in the process often have a reactive functional group). In addition, in case where halogen compounds are used in the process of preparing the organic compounds, unreacted intermediates will remain in the reaction system, and the starting halogen compounds not reacted completely will also remain therein. These unreacted intermediates and halogen compounds will be impurities in the organic compounds prepared. Moreover, in case where halogen solvents are used in the process of preparing the organic compounds, the organic compounds prepared will be often contaminated by halogen (e.g., chlorine)-added olefins, or, depending on the reaction condition, by oxidized organic compounds.
Accordingly, the organic compound materials are, immediately after having been prepared through chemical synthesis, often contaminated by impurities of various compounds. Of all such impurities, we, the present inventors have found that halogen compounds are the most serious, as significantly attenuating emission luminance and shortening the emission life. Many impurities mentioned above contain halogen atoms acting as a reactive functional group, and they trap holes and electrons that have moved from the electrodes into the organic compound layers. Accordingly, the acceptable uppermost limit of the halogen-containing impurities that may be in the organic compound materials is 500 ppm. We have found that organic EL devices in which the organic compound layers are formed from materials having a halogen compound content higher than the uppermost limit are often significantly confronted with the problems of emission luminance attenuation and short emission life.
Just after having been prepared through chemical synthesis, organic compound materials are often contaminated with impurities, which are described more concretely. For example, in chemical synthesis of producing styryl compounds to be represented by the following reaction formula: 
wherein Ar, Arxe2x80x2, X and Y each represent an aryl group; and
Z represents a halogen atom,
halogen compounds of the following formulae: 
wherein Ar, Arxe2x80x2, X and Y each represent an aryl group; and
Z represents a halogen atom.
exist as impurities in the reaction system. Accordingly, organic compound materials comprising the styryl compounds prepared through the reaction mentioned above are contaminated with the impurities of such halogen compounds.
In chemical synthesis of producing amine compounds to be represented by the following reaction formula: 
wherein Ar, Arxe2x80x2 and Arxe2x80x3 each represent an aryl group; and
Z represents a halogen atom,
halogen compounds of the following formulae: 
wherein Ar, Arxe2x80x2 and Arxe2x80x3 each represent an aryl group; and
Z represents a halogen atom,
and also amine oxides exist as impurities in the reaction system. Accordingly, organic compound materials comprising the amine compounds prepared through the reaction mentioned above are contaminated with the impurities of such halogen compounds and amine oxides. These are examples of intermediates and precursors in the process of chemical synthesis.
Amine compounds may be prepared through the following reaction, which also gives some impurities to be mentioned below. 
In these processes, the products produced will be contaminated with the following impurities, intermediates or precursors. 
wherein Ar, Arxe2x80x2 and Arxe2x80x3 each represent an aryl group; and Z represents a halogen atom.
Polyfluorenone compounds will be contaminated with the following impurities: 
wherein Z represents a halogen atom.
In case where various organic compounds for use herein are produced in other processes of chemical reaction, the organic compounds produced should be so controlled that their impurity content is smaller than 1000 ppm, including 0 ppm. The impurities of the organic compounds may be derived from the starting substances used, or may be intermediates or precursors formed in the process of chemical reaction to give the organic compounds, or may be such intermediates or precursors having a reactive functional group (including, for example, halogens, amino, hydroxy and carboxyl groups, etc.).
Various methods are known for quantifying the impurity content of the organic compound materials purified in various purification methods such as those mentioned above. In the invention, the purified organic compound materials are analyzed through high-performance liquid chromatography to quantify the impurities therein, and those of which the impurity content is lower than the predetermined value as above are selectively used for forming the organic compound layers of the organic EL device of the invention. For selecting the organic compound materials for use herein, high-performance liquid chromatographyis preferred to any other methods. This is because organic compound materials suitable to the invention are selected more rapidly and more accurately in the method of high-performance liquid chromatography than in any other methods.
In the method of high-performance liquid chromatography, the mobile phase is moved in the column by the power of a high-pressure pump (pressure: 350 to 500 kg/cm2). In the method, therefore, the time for separation is short, and rapid quantification is possible. The filler for the method comprises porous particles all having a small grain size of from 5 to 10xcexc and having a large surface area, and therefore has good separation capability. The column can be connected with a high-sensitivity detector, in which accurate analysis is possible. In addition, since the flow rate through the column can be kept constant all the time, the method of high-performance liquid chromatography ensures good reproducibility. Some typical examples of the filler and the separation mode for the method of high-performance liquid chromatography are shown in Table 1.
In the method of high-performance liquid chromatography, the separation mode varies, depending on the fixed phase and the mobile phase, and may be any desired one.
For quantifying the impurities in the organic compound materials to be used herein for forming organic compound layers of the organic EL device of the invention, preferred is reversed-phase chromatography as its separation efficiency is good. Reversed-phase chromatography is a type of partitioning chromatography, and the filler used therein is ODS (octadecyl-bonding silica) which is a type of chemical-bonding silica gel. Such an ODS filler is a typical one for high-performance liquid chromatography, and is applicable to a broad range of various compounds. The solvent for reversed-phase chromatography may be a polar solvent including methanol, acetonitrile and others, or may also be a mixed solvent of water and such a polar solvent. Especially preferred is acetonitrile.
Any detector is usable in high-performance liquid chromatography, including, for example, an ultraviolet absorptiometer (UV), a differential refractometer (RI), etc. Preferred is an ultraviolet absorptiometer (w), since the base line for the data detected therein is stable, and the data are not influenced by the ambient temperature and the flow rate, therefore ensuring high-sensitivity detection.
Accordingly, the best combination of the filler, the solvent and the detector for high-performance liquid chromatography to be employed herein is as follows. The filler is ODS; the solvent is acetonitrile for reversed-phase chromatography; and the detector is an ultraviolet absorptiometer (UV).
In case where the organic compound materials to be used for forming the layers of the organic EL device of the invention are soluble in the solvent, acetonitrile, there occurs no problem in analyzing the materials through high-performance liquid chromatography with the solvent, acetonitrile. However, in case where the materials are hardly soluble in the solvent, acetonitrile, they must be processed as follows: The material to be analyzed is first dissolved in a solvent capable of dissolving it, and then a bad solvent, for example, methanol or a mixed solvent of methanol and water is added to the resulting solution for re-precipitating it. Next, the insoluble solid is taken out through filtration, and the solvent is completely evaporated away by the use of an evaporator. Acetonitrile is added to this to prepare a sample solution of the material in acetonitrile. In that manner, the organic compound materials hardly soluble in acetonitrile can be analyzed through high-performance liquid chromatography with acetonitrile.